This invention relates to a hydraulic control system for controlling the operations of a plurality of actuators.
A civil engineering machine (e.g., a power shovel) or other machine including a plurality of actuators is equipped with a hydraulic control system. One such conventional hydraulic control system as described in the prior art section of the specification of Japanese Laid-Open (Kokai) Patent Application No. 11706/85 comprises one pump of a large capacity, directional control valves corresponding respectively to the actuators, and flow control valves of the pressure compensating type (hereinafter referred to as "pressure compensation valves"). Each pressure compensation valve is connected between the pump and its mating directional control valve.
Each directional control valve has two actuator ports connected to the actuator, and a spool which can be moved through an external operation. When the spool is moved from its neutral position in one direction, one of the actuator ports is selectively communicated with the pressure compensation valve, so that oil fed from the pump is supplied to the actuator via the pressure compensation valve and the selected actuator port, thereby driving the actuator in one direction. When the spool of the directional control valve is moved in the opposite direction, the other actuator port is communicated with the pressure compensation valve, so that the actuator is driven in the opposite direction. The directional control valve has a throttle portion which varies in the degree of opening (i.e., the cross-sectional area of flow) in accordance with the position of the spool.
Each pressure compensation valve comprises a balance piston, a spring urging the balance piston, and a throttle portion whose degree of opening is controlled by the balance piston. A pressure (pressure supplied to the directional control valve from the pressure compensation valve) in a passage extending between the throttle portion of the pressure compensation valve and the throttle portion of the directional control valve is applied to the balance piston, and also a load pressure produced in the actuator is applied to the balance piston. These two pressures act on the balance piston in opposite directions. The position of the balance piston and hence the degree of opening of the throttle portion of the pressure compensation valve are so determined that a pressure differential across the balance piston (i.e., a difference between these two pressures) can be kept at a predetermined target value or level. This target value is determined by the spring force of the above spring.
Thus, with the use of the pressure compensation valve, irrespective of the load pressure produced in each actuator, the actuator receives an amount of the oil (per unit time) corresponding to the degree of opening of the throttle portion determined by the position of the spool of the directional control valve.
When the total amount of the oil per unit time, required by the actuators operating at the same time, becomes too large, the ability of the pump to output the oil becomes inadequate, so that the pump pressure decreases. At this time, the pressure compensation valves fully open the throttle so that the difference between the supply pressure supplied from the pressure compensation valve and the load pressure can be increased to the target value determined by the spring (Actually, this difference does not reach this target value). As a result, the pressure compensation valves lose their pressure compensation function, so that those of the actuators receiving relatively small loads are driven whereas the other actuators receiving relatively heavy loads are not driven.
The hydraulic control system shown in the drawings of the above Japanese Laid-Open Patent Application No. 11706/85 overcomes the above problems. In this conventional hydraulic control system, the maximum load pressure is applied to the balance pistons of all of the pressure compensation valves in the direction to close the throttles of the pressure compensation valves, and at the same time the pressure of the pump is applied to the balance pistons in the direction to open the throttles of the pressure control valves. Here, "the maximum load pressure" means the greatest load pressure among the load pressures produced in the plurality of actuators. The force produced due to a difference between the pump pressure and the maximum load pressure is used instead of the force produced by the aforesaid spring. In this hydraulic control system, when the total amount of the oil required by the actuators per unit time exceeds the ability of the pump to output the oil to decrease the pump pressure so that the difference between the pump pressure and the maximum load pressure decreases, the difference between the load pressure and the supply pressure supplied from the pressure compensation valves decreases in all the pressure compensation valves. As a result, the amounts of supply of the oil per unit time to those of the actuators which are being driven are reduced at the same rate. In this condition, the throttle portion of the pressure compensation valve corresponding to the actuator subjected to the maximum load pressure is fully opened, and the throttling functions of the other pressure compensation valves are secured, and hence those of the actuators corresponding to those of the directional control valves in their operative condition can be all driven irrespective of the magnitude of the load.
In order to determine the maximum load pressure among the above load pressures, there are used shuttle valves the number of which is less by one than the number of the plurality of actuators.
Other hydraulic control systems of the type, which include directional control valves, pressure compensation valves and check valves replacing shuttle valves (each group of valves correspond in number to the actuators), are disclosed in U.S. Pat. No. 4,739,617, West German Patent No. DE 36 44 737, and PCT application of Japan origin filed by one of the two Applicants of the present application (International Filing Date: July 7, 1989; Designated countries: U. S. A., Europe, etc.)
The above-mentioned hydraulic control systems have been proposed as relatively abstract hydraulic circuits, and a practical hydraulic control system of the type in which directional control valves, pressure compensation valves and shuttle valves are incorporated or mounted in a body have not yet been developed. The other of the two Applicants of the present application has filed Japanese Utility Model Application No. 46811/88 on Apr. 8, 1988 and directed to such a hydraulic control apparatus. It is expected that this Japanese utility model application will be laid open to public inspection on October or November, 1989.
Incidentally, Japanese Patent Publication No. 10707/86, Japanese Laid-Open Patent Application No. 110884/82, and U.S. Pat. No. 4,856,549 (filed by the other of the two Applicants of the present application) disclose the prior art which incorporate or contain at least one group of directional control valves, pressure compensation valves and shuttle valves in a body, although these prior art are different from the above-mentioned hydraulic control system.